The present invention relates to a mobile terminal for use in a mobile communication system. Specifically, the present invention relates to a control of the handing over of the communication channel so as to continue the communication, when a mobile terminal moves from a base station cell of a current base radio station, which is communicating with the mobile terminal, to the base station cell of another base radio station. In a handing over procedure, a supplemental communication channel is provided to the communication channel of the new base radio station.
In a mobile communication system, when a mobile terminal moves from the base station cell of a communicating base radio station into the cell of a neighboring base radio station, it is necessary to hand over the communication channel to the neighboring base radio station, and to provide a supplemental channel to the neighboring base radio station so as to continue the communication.
Japanese Patent Application JP-A-9-37327 discloses an example of the control of handing over in the prior art mobile communication system. FIG. 1 is a schematic diagram of the control of handing over disclosed therein. Reference numerals 1a, 1b, 1c denote base radio stations, each of which covers a base station cell 2a, 2b, 2c. Reference numeral 3 denotes a mobile terminal, which is moving in the area of the base station cells 2a, 2b, and 2c. The mobile terminal is communicating with one of the base radio stations.
FIG. 2 is a flow chart showing the control of the handing over. At first, the mobile terminal 3 measures the level and the qualities of communication of the input signal received from the currently communicating base radio station 1a (step ST 1). The qualities of communication include, for example, a bit error rate and a frame error rate. Then the mobile terminal judges whether the received input level and the qualities of the communication are higher than a predetermined value (step St 2). When they are higher than predetermined values, the procedure returns to the step St 1. On the other hand, when they are less than the predetermined values, the mobile terminal measures the level and the qualities of communication of the input signals received from the other base radio stations 1b, 1c. (step St 3).
When the level of the input signal and the qualities of communication of the input signal received therefrom is higher than a predetermined value, the mobile terminal measures the tendency of changing rate of the level and the qualities of communication of the input signal received from the base radio stations, and judges whether they are increasing or decreasing (step ST 4).
Then the mobile terminal judges whether there are base stations, the tendency of changing rate of the level or the qualities of communication of the input signal therefrom is increasing (step ST 5). When there are at least one base radio station, which has such an increasing tendency, the mobile terminal judges whether a plurality of the radio base stations have such an increasing tendency or only one base radio station has such an increasing tendency (step ST 6).
When only one base station has such an increasing tendency, the communication is handed over to the base radio station (step ST 7). FIG. 1 (a) shows such a situation. In this case, the communication channel is handed over to the base station cell 2c of the base radio station 1c. On the other hand, when a plurality of base radio stations have an increasing tendency, the communication channel is handed over to the base radio station, the level of the input signal or the qualities of communication of the input signal received therefrom is the lowest among them (step ST 8). FIG. 1 (b) shows such a situation. In this case, the communication channel is handed over to the base station cell 2c of the base radio station 1c. 
When it is judged that no base radio station has such an increasing tendency in the step ST 5, the communication channel is handed over to the base station cell of the base radio station, the level of the input signal or the qualities of communication of the input signal received therefrom is the highest (step ST 9). FIG. 1(c) shows such a situation. In this case, the communication channel is handed over to the base station cell 2c of the base radio station 1c. 
Japanese patent application, JP-A-6-292258, discloses another example of the control of handing over in a mobile communication system, in which each of the mobile terminals in the mobile communication system has a function to generate a data for measuring the bit error rate (BER) of the input signal received by the mobile terminal. Each of the base radio stations has a function to receive the data to measure the bit error rate BER, so that the communication channel is handed over to the base station cell of the base radio station, the bit error rate BER of which is the smallest.
In general, it is known that a plurality of communication paths appear between a base radio station and a mobile terminal due to the diffraction and/or the reflection of the electromagnetic wave, and a multi-fading effect due to a plurality of communication paths is observed. In such a circumstance, the level of the input signal received by the mobile terminal shows a Rayleigh""s distribution, as shown in FIG. 3. For avoiding the influence of the fading effect, a temporal mean value of the instantaneous level of the input signals in a certain time interval is often adopted as the level of the input signal. However, because the Rayleigh""s distribution is a function of the spatial position, the change of the instantaneous measured level of the input signal depends on the state of the mobile terminal, for example, the velocity and direction of the movement. Therefore, it is difficult to eliminate the influence of the fading effect using such a temporal mean value.
When a moving vehicle, in which a mobile terminal is equipped, moves at a high velocity, it is necessary to shorten the smoothing time of the input signal, and the temporal resolution of the input signal has to be increased, in order to follow the rapid change of the level and/or the qualities of communication of the input signal. On the other hand, when the moving vehicle moves at a low velocity or is nearly stopping, the time for smoothing the input signal has to be long so as to increase the preciseness of the measurement. That is because a simultaneous actuation of a plurality of channels for handing over has to be prevented. Such simultaneous actuation can take place at a marginal area of a base station cell of a base radio station, due to the chattering phenomenon of the input level or qualities of communication of the input signal. However, the mobile terminal of a mobile communication system in the prior art has no means to detect the velocity of the moving object.
When a moving vehicle, in which a mobile terminal is equipped, moves at a high velocity, it is necessary to shorten the smoothing time of the input signal, and the temporal resolution of the input signal has to be increased, in order to follow the rapid change of the level and/or the qualities of communication of the input signal. On the other hand, when the moving vehicle moves at a low velocity or is nearly stopping, the time for smoothing the input signal has to be long so as to increase the preciseness of the measurement. That is because a simultaneous actuation of a plurality of channels for handing over has to be prevented. Such simultaneous actuation can takes place at a marginal area of a base station cell of a base radio station, due to the chattering phenomenon of the input level or qualities of communication of the input signal. However, the mobile terminal of a mobile communication system in the prior art has no means to detect the velocity of the moving object.
It is well known that the frequency of an electromagnetic wave measured in a moving object is shifted compared to the frequency measured in a static system due to the Doppler effect. The frequency f0 measured in a moving system can be expressed as follows:
f0=fxe2x88x92kxc2x7vxe2x80x83xe2x80x83(1)
where f is the frequency measured in a static system, v is the velocity vector of the moving system, and k is the wave number vector of the electromagnetic wave.
The clock in a mobile terminal is deviated from that of the base radio station. Thus, in such a mobile communication system, the frequency of the mobile terminal is pulled to the frequency of the clock of the base radio station so as to receive the electromagnetic signal from the base radio station. Thus, the frequency deviation xcex94f of the measured signal includes the frequency deviation foffset between the clocks as well as the frequency deviation due to the Doppler effect:
xcex94f=foffsetxe2x88x92kxc2x7vxe2x80x83xe2x80x832)
In a TDMA type communication system, a time slot is allocated to each of the communicating mobile terminals. The time slots allocated to a plurality of mobile terminals can overlap in the base radio station due to the delay of the propagation of electromagnetic wave. For avoiding the overlapping, the base radio station controls finely the signal sending timing from each of the mobile terminals, by setting a time alignment, which instructs the change of the sending timing from each of the mobile terminal. That is to say, a mobile terminal found in a far point from the base radio station begins to send signals at an advanced timing compared to a mobile terminal found in a nearer point to the base radio station
The starting time to send signals from a mobile terminal is determined on the basis of the receiving time of the signal from the base radio station, and is changed by the time alignment, which instructs how long the sending timing shall be advanced. Therefore, the distance between the mobile terminal and the base radio station can be detected from the time difference between the receiving timing and the sending timing of the signal. When the mobile terminal is moving, the time alignment is changed at every moment. And when the mobile terminal is nearly stopping, the time alignment is almost constant.
By the way, there is a relation between the proportion Eb/NO, which is a proportion of bit energy to noise density, and the bit error rate BER. An example of the relation is shown in FIG. 4. This relation depends only on the characteristics of the receiver, and does not depend on the environment. According to this relation, even when the electric field of the received electromagnetic wave is strong, if the noise contained in the received signal is large, a small proportion Eb/N0 and a large bit error rate BER are observed.
The qualities of a communication channel can be substantially estimated by the bit error rate BER. However, in a mobile communication system, the measurement of the bit error rate BER is difficult. Thus, the proportion Eb/N0 is often employed in place of the bit error rate BER. However, in this case, even when the level of the received signal is large, if the intensity of the interference wave or noise are still larger, and the mobile communication system has no means to measure the moving velocity of the moving vehicle, it is difficult to adjust the smoothing time of the input signal, corresponding to the moving velocity of the moving vehicle. Thus, it is difficult to avoid the simultaneous actuation of a plurality of the handing over procedures.
The qualities of the communication channel can be evaluated according to the level of the electric field of the received signal. In such a case, however, even when the level of the received signal is large, if the intensity of the interference wave or noise are still larger, the signal to noise ratio S/N decreases apparently. Thus, in such a case, the qualities of a communication channel is evaluated to be rather low contrary to the actuality.
An object of the present invention is to eliminate these problems.
Another object is to propose a mobile terminal for a mobile communication system, comprising a hand over controlling means, which performs a procedure to select the best base radio station, even when the mobile terminal is in a fading circumstance, so that the number of the handing over actuation times is reduced as least as possible, so as to avoid the deterioration of the qualities of communication and to reduce the traffic of the controlling signals.
In an embodiment of the present invention, the mobile terminal for a mobile communication system comprises: an electric field level measuring means for measuring the level of the electric field of the signal received from the base radio station; a bit error rate measuring means for measuring the bit error rate of the signal received from the base radio station; an electric field correcting means for correcting the level of the electric field measured by the electric field level measuring means, on the basis of the bit error rate measured by the bit error rate measuring means. And the handing over between the radio communication channels is controlled on the basis of the electric field corrected by the electric field correcting means.
According to this structure, the qualities of communication between the mobile terminal and the base radio station after the handing over is improved, and the continuous actuation of the handing over procedures seldom takes place. Thus the increase of the traffic due to the continuous actuation of the handing over procedures can be prevented.
In another embodiment of the present invention, the mobile terminal for a mobile communication system further comprises: a reference pattern comparing means for comparing the reference pattern data portion, which is embedded in the data received from the base radio station, with a reference pattern stored in the mobile terminal, so as to detect the number of the error bits contained in the reference pattern data portion; and an error correcting means for correcting the user data portion in the received data and for detecting the number of the corrected bits; wherein the bit error rate measuring means measures the bit error rate of the receive signal, on the basis of the number of the error bits in the reference pattern data portion, which is measured by the reference pattern comparing means, and the number of the corrected bits in the user data portion, which is detected by the error correcting means.
In another embodiment of the present invention, the mobile terminal for a mobile communication system comprises: an electric field level measuring means for measuring the level of the electric field of the signal received from the base radio station; a mobile terminal velocity measuring means for measuring the velocity of the mobile terminal with respect to the base radio station. And the handing over between the radio communication channels is controlled on the basis of the velocity measured by the mobile terminal velocity measuring means and the level of the electric field of the received signal, which is measured by the electric field level measuring means.
As a result, the qualities of communication after the handing over is improved. And a continuous actuation of the handing over procedures seldom takes place. Thus, the increase of the traffic due to the continuous actuation of the handing over can be prevented.
In another embodiment of the present invention, the mobile terminal for a mobile communication system comprises: an electric field forecasting means for forecasting the level of the electric field of the signal to be received in the future, on the basis of the level of the electric field measured by the electric field measuring means and the moving velocity of the mobile terminal with respect to the base radio station, which is measured by a mobile terminal velocity measuring means. The handing over between the radio communication channels is controlled, using the level of the electric field forecasted by the electric field forecasting means.
It is easy to perform a procedure to control the handing over to select the base radio station, the forecasted level of the electric field of the signal to be received therefrom is the highest, as the base radio station to be handed over.
In another embodiment of the present invention, the mobile terminal for a mobile communication system further comprises: a Doppler frequency detecting means for detecting the Doppler frequency of the signal received from the base radio station. And a mobile terminal velocity measuring means measures the moving velocity of the mobile terminal with respect to the base radio station, on the basis of the Doppler frequency, which is detected by the Doppler frequency detecting means.
The fact that the Doppler frequency detected by the Doppler frequency detecting means is xe2x80x9cpositivexe2x80x9d means that the mobile terminal is moving towards the base radio station, and the fact that the Doppler frequency is xe2x80x9cnegativexe2x80x9d means that the mobile terminal is moving in the direction to separate from the base radio station. The absolute value of the Doppler frequency is proportional to the moving velocity of the mobile terminal in the propagation direction of the electromagnetic wave.
The Doppler frequency can be measured without being influenced by the fading effect. Thus, it is possible to detect precisely whether the mobile terminal is approaching to the base radio station.
In another embodiment of the present invention, the mobile terminal for a mobile communication system comprises: an automatic frequency controlling means, which introduces in itself the frequency of the signal received from the base radio station. And the Doppler frequency detecting means detects the Doppler frequency, from the frequency deviation between the frequency introduced in by the automatic frequency controlling means and the clock frequency of the mobile terminal.
In another embodiment of the present invention, the mobile terminal for a mobile communication system further comprises: a propagation delay detecting means for detecting the propagation delay of the electromagnetic wave between the mobile terminal and the base radio station; and a distance detecting means for detecting the distance between the mobile terminal and the base radio station, on the basis of the propagation delay detected by the propagation delay detecting means. And the mobile terminal velocity measuring means measures the velocity of the mobile terminal with respect to the base radio station, on the basis of the time variation of the distance detected by the distance detecting means.
In this case, the qualities of communication between the mobile terminal and the base station after the handing over is improved. And a continuous actuation of the handing over procedures seldom takes place. Thus the increase of traffic due to the handing over can be prevented.
In another embodiment of the present invention, the mobile terminal for a mobile communication system comprises a timing comparator which compares the timing of the signal sending from the mobile terminal and the timing of signal receiving from the base radio station so as to detect the difference between them. And the distance between the mobile terminal and the base radio station is measured, on the basis of the detected difference between the timing of the signal sending from the mobile terminal and the timing of signal receiving from the base radio station. In this case, the detected distance between the mobile terminal and the base radio station is hardly influenced by the fading effect. Thus, it is possible to detect precisely whether the mobile terminal is approaching to the base radio station or not.